Gymnostomum, a genus of 20 species (Crosby et al. 2000), is found on rocks, boulders and cliff faces usually in wet or seepy places. It is generally associated with calcareous habitats but also occurs occasionally on acidic rocks. The genus usually is soft-green in color, has lanceolate leaves with plane leaf margins that are roughened papillose above and at times serrulate below. Its stem has a central strand and at times a weakly developed hyalodermis is present. The leaf cells in Gymnostomum are firm-walled throughout and are ornamented by simple, low, scattered papillae. Although most members of Gymnostomum have two stereid bands in the costa, Central American collections of G. aeruginosum have only a single (dorsal) stereid band. The perigonia and perichaetia are terminal in Gymnostomum and its capsules are eperistomate.

Zander (1993) considered Gymnostomum a reduced, hydrophilic segregate of Barbula. The genus however is often associated with Hymenostylium recurvirostrum. Zander (1993) puts Gymnostomum and Hymenostylium in different tribes on the basis of whether or not a stem central strand is present. Some forms of Molendoa can be difficult to distinguish from Gymnostomum. Usually the two genera can be distinguished by the percurrent to excurrent costa, massive leaf papillae and lateral perichaetia of Molendoa. Zander (1977) provided an in-depth treatment of the genus in Middle America.